The Many Moods of Titan

A set of recent papers, many of which draw on data from NASA's Cassini spacecraft, reveal new details in the emerging picture of how Saturn's moon Titan shifts with the seasons and even throughout the day. The papers, published in the journal Planetary and Space Science in a special issue titled "Titan through Time," show how this largest moon of Saturn is a cousin -- though a very peculiar cousin -- of Earth.

"As a whole, these papers give us some new pieces in the jigsaw puzzle that is Titan," said Conor Nixon, a Cassini team scientist at the NASA Goddard Space Flight Center, Greenbelt, Md., who co-edited the special issue with Ralph Lorenz, a Cassini team scientist based at the Johns Hopkins University Applied Physics Laboratory, Laurel, Md. "They show us in detail how Titan's atmosphere and surface behave like Earth's -- with clouds, rainfall, river valleys and lakes. They show us that the seasons change, too, on Titan, although in unexpected ways."

A paper led by Stephane Le Mouelic, a Cassini team associate at the French National Center for Scientific Research (CNRS) at the University of Nantes, highlights the kind of seasonal changes that occur at Titan with a set of the best looks yet at the vast north polar cloud.

A newly published selection of images -- made from data collected by Cassini's visual and infrared mapping spectrometer over five years -- shows how the cloud thinned out and retreated as winter turned to spring in the northern hemisphere.

Cassini first detected the cloud, which scientists think is composed of ethane, shortly after its arrival in the Saturn system in 2004. The first really good opportunity for the spectrometer to observe the half-lit north pole occurred on December 2006. At that time, the cloud appeared to cover the north pole completely down to about 55 degrees north latitude. But in the 2009 images, the cloud cover had so many gaps it unveiled to Cassini's view the hydrocarbon sea known as Kraken Mare and surrounding lakes.

"Snapshot by snapshot, these images give Cassini scientists concrete evidence that Titan's atmosphere changes with the seasons," said Le Mouelic. "We can't wait to see more of the surface, in particular in the northern land of lakes and seas."

In data gathered by Cassini's composite infrared mapping spectrometer to analyze temperatures on Titan's surface, not only did scientists see seasonal change on Titan, but they also saw day-to-night surface temperature changes for the first time. The paper, led by Valeria Cottini, a Cassini associate based at Goddard, used data collected at a wavelength that penetrated through Titan's thick haze to see the moon's surface. Like Earth, the surface temperature of Titan, which is usually in the chilly mid-90 kelvins (around minus 288 degrees Fahrenheit), was significantly warmer in the late afternoon than around dawn.

"While the temperature difference -- 1.5 kelvins -- is smaller than what we're used to on Earth, the finding still shows that Titan's surface behaves in ways familiar to us earthlings," Cottini said. "We now see how the long Titan day (about 16 Earth days) reveals itself through the clouds."

A third paper by Dominic Fortes, an outside researcher based at University College London, England, addresses the long-standing mystery of the structure of Titan's interior and its relationship to the strikingly Earth-like range of geologic features seen on the surface. Fortes constructed an array of models of Titan's interior and compared these with newly acquired data from Cassini's radio science experiment.

The work shows the moon's interior is partly or possibly even fully differentiated. This means that the core is denser than outer parts of the moon, although less dense than expected. This may be because the core still contains a large amount of ice or because the rocks have reacted with water to form low-density minerals.

Earth and other terrestrial planets are fully differentiated and have a dense iron core. Fortes' model, however, rules out a metallic core inside Titan and agrees with Cassini magnetometer data that suggests a relatively cool and wet rocky interior. The new model also highlights the difficulty in explaining the presence of important gases in Titan's atmosphere, such as methane and argon-40, since they do not appear to be able to escape from the core.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory manages the mission for NASA's Science Mission Directorate, Washington, D.C. The visual and infrared mapping spectrometer team is based at the University of Arizona, Tucson. The composite infrared spectrometer team is based at NASA's Goddard Space Flight Center in Greenbelt, Md., where the instrument was built. The radio science subsystem has been jointly developed by NASA and the Italian Space Agency.

Chemical Clues On Formation of Planetary Systems: Earth 'Siblings' Can Be Different

An international team of researchers, with the participation of IAC astronomers, has discovered that the chemical structure of Earth-like planets can be very different from the bulk composition of Earth. This may have a dramatic effect on the existence and formation of the biospheres and life on Earth-like planets.

The study of the photospheric stellar abundances of the planet-host stars is the key to understanding how protoplanets form, as well as which protoplanetary clouds evolve planets and which do not. These studies, which have important implications for models of giant planet formation and evolution, also help us to investigate the internal and atmospheric structure and composition of extrasolar planets..
Theoretical studies suggest that C/O and Mg/Si, are the most important elemental ratios in determining the mineralogy of terrestrial planets, and they can give us information about the composition of these planets. The C/O ratio controls the distribution of Si among carbide and oxide species, while Mg/Si gives information on the silicate mineralogy. In 2010 Bond et al. (2010b) carried out the first numerical simulations of planet formation in which the chemical composition of the proto-planetary cloud was taken as an input parameter. Terrestrial planets were found to form in all the simulations with a wide variety of chemical compositions so these planets might be very different from Earth.
Delgado Mena et al. (2010) have carried out the first detailed and uniform study of C, O, Mg and Si abundances for 61 stars with detected planets and 270 stars without detected planets from the homogeneous high-quality unbiased HARPS GTO sample. They found mineralogical ratios quite different from those in the Sun, showing that there is a wide variety of planetary systems which are unlike the Solar System. Many planetary-host stars present a Mg/Si value lower than 1, so their planets will have a high Si content to form species such as MgSiO3. This type of composition can have important implications for planetary processes like plate tectonics, atmospheric composition and volcanism.
'There could be billions of Earth-like planets in the Universe but a great majority of them may have a totally different internal and atmospheric structure. Building planets in chemically non-solar environments (which are very common in the Universe) may lead to the formation of strange worlds, very different from the Earth! The amount of radioactive and some refractory elements (especially Si) may have drastic implications for planetary processes such as plate tectonics and volcanic activity,' concludes Garik Israelian.
The latest numerical simulations have shown that a wide range of extrasolar terrestrial planet bulk compositions are likely to exist. Planets simulated as forming around stars with Mg/Si ratios less than 1 are found to be Mg-depleted (compared to Earth), consisting of silicate species such as pyroxene and various types of feldspars. Planetary carbon abundances also vary in accordance with the host stars' C/O ratio. The predicted abundances are in keeping with observations of polluted white dwarfs (expected to have accreted their inner planets during their previous red giant stage).
'The observed variations in the key C/O and Mg/Si ratios for known planetary host stars implies that a wide variety of extrasolar terrestrial planet compositions are likely to exist, ranging from relatively "Earth-like" planets to those that are dominated by C, such as graphite and carbide phases (e.g. SiC, TiC),' Delgado Mena stresses.
The results of Delgado Mena et al. (2010) were used in this study as they are the first to determine the abundance of all of the required elements in a completely internally consistent manner, using high quality spectra and an identical approach for all stars and elements, for a large sample of both host and non-host stars.
The chemical and dynamical simulations were combined by assuming that each embryo retains the composition of its formation location and contributes the same composition to the simulated terrestrial planet. The innermost terrestrial planets (located within ?0.5 AU from the host star) contain a significant amount of the refractory elements Al and Ca (?47% of the planetary mass). Planets forming beyond ?0.5 AU from the host star contain steadily less Al and Ca with increasing distance. One planetary system, 55 Cnc, has a C/O ratio above 1 (C/O = 1.12). This system produced carbon-enriched "Earth-like" planets. All of the terrestrial planets considered in this work have compositions dominated by O, Fe, Mg and Si, most of these elements being delivered in the form of silicates or metals (in the case of iron). However, important differences between those planets forming in systems with C/O < 0.8 (HD17051, HD19994) and those with C/O > 0.8 (55Cnc) have been found.
'We are working hard to decrease abundance measurement errors and make the results of theoretical models and numerical simulations more reliable,' comments González Hernández, 'There is much work to be done'.

Black Hole Came from a Shredded Galaxy

Astronomers using NASA's Hubble Space Telescope have found a cluster of young, blue stars encircling the first intermediate-mass black hole ever discovered. The presence of the star cluster suggests that the black hole was once at the core of a now-disintegrated dwarf galaxy. The discovery of the black hole and the star cluster has important implications for understanding the evolution of supermassive black holes and galaxies.


"For the first time, we have evidence on the environment, and thus the origin, of this middle-weight black hole," said Mathieu Servillat, who worked at the Harvard-Smithsonian Center for Astrophysics when this research was conducted.
Astronomers know how massive stars collapse to form stellar-mass black holes (which weigh about 10 times the mass of our sun), but it's not clear how supermassive black holes (like the four million solar-mass monster at the center of the Milky Way) form in the cores of galaxies. One idea is that supermassive black holes may build up through the merger of smaller, intermediate-mass black holes weighing hundreds to thousands of suns.
Lead author Sean Farrell, of the Sydney Institute for Astronomy in Australia, discovered this unusual black hole in 2009 using the European Space Agency's XMM-Newton X-ray space telescope. Known as HLX-1 (Hyper-Luminous X-ray source 1), the black hole weighs in at 20,000 solar masses and lies towards the edge of the galaxy ESO 243-49, which is 290 million light-years from Earth.
Farrell and his team then observed HLX-1 simultaneously with NASA's Swift observatory in X-ray and Hubble in near-infrared, optical, and ultraviolet wavelengths. The intensity and the color of the light shows a cluster of young stars, 250 light-years across, encircling the black hole. Hubble can't resolve the stars individually because the suspected cluster is too far away. The brightness and color are consistent with other clusters of young stars seen in other galaxies.
Farrell's team detected blue light from hot gas in the accretion disk swirling around the black hole. However, they also detected red light produced by much cooler gas, which would most likely come from stars. Computer models suggested the presence of a young, massive cluster of stars encircling the black hole.
"What we can definitely say with our Hubble data is that we require both emission from an accretion disk and emission from a stellar population to explain the colors we see," said Farrell.
Such young clusters of stars are commonly seen in nearby galaxies, but not outside the flattened starry disk, as found with HLX-1. The best explanation is that the HLX-1 black hole was the central black hole in a dwarf galaxy. The larger host galaxy then captured the dwarf. Most of the dwarf's stars were stripped away through the collision between the galaxies. At the same time, new young stars were formed in the encounter. The interaction that compressed the gas around the black hole also triggered star formation.
Farrell and Servillat found that the star cluster must be less than 200 million years old. This means that the bulk of the stars were formed following the dwarf's collision with the larger galaxy. The age of the stars tells how long ago the two galaxies crashed into each other.
The future of the black hole is uncertain at this stage. It depends on its trajectory, which is currently unknown. It's possible the black hole may spiral in to the center of the big galaxy and eventually merge with the supermassive black hole there. Alternately, the black hole could settle into a stable orbit around the galaxy. Either way, it's likely to fade away in X-rays as it depletes its supply of gas.
"This black hole is unique in that it's the only intermediate-mass black hole we've found so far. Its rarity suggests that these black holes are only visible for a short time," said Servillat.
More observations are planned this year to track the history of the interaction between the two galaxies.
The new findings are being published in the February 15 issue of the Astrophysical Journal Letters.

Great Eruption Replay: Astronomers Watch Delayed Broadcast of Powerful Stellar Eruption

Astronomers are watching a delayed broadcast of a spectacular outburst from the unstable, behemoth double-star system Eta Carinae, an event initially seen on Earth nearly 170 years ago.




Dubbed the "Great Eruption," the outburst first caught the attention of sky watchers in 1837 and was observed through 1858. But astronomers didn't have sophisticated science instruments to accurately record the star system's petulant activity.
Luckily for today's astronomers, some of the light from the eruption took an indirect path to Earth and is just arriving now, providing an opportunity to analyze the outburst in detail. The wayward light was heading in a different direction, away from our planet, when it bounced off dust clouds lingering far from the turbulent stars and was rerouted to Earth, an effect called a "light echo." Because of its longer path, the light reached Earth 170 years later than the light that arrived directly.
The observations of Eta Carinae's light echo are providing new insight into the behavior of powerful massive stars on the brink of detonation. The views of the nearby erupting star reveal some unexpected results, which will force astronomers to modify physical models of the outburst.
"When the eruption was seen on Earth 170 years ago, there were no cameras capable of recording the event," explained the study's leader, Armin Rest of the Space Telescope Science Institute in Baltimore, Md. "Everything astronomers have known to date about Eta Carinae's outburst is from eyewitness accounts. Modern observations with science instruments were made years after the eruption actually happened. It's as if nature has left behind a surveillance tape of the event, which we are now just beginning to watch. We can trace it year by year to see how the outburst changed."
The team's paper will appear Feb. 16 in a letter to the journalNature.
Located 7,500 light-years from Earth, Eta Carinae is one of the largest and brightest star systems in our Milky Way galaxy. Although the chaotic duo is known for its petulant outbursts, the Great Eruption was the biggest ever observed. During the 20-year episode, Eta Carinae shed some 20 solar masses and became the second brightest star in the sky. Some of the outflow formed the system's twin giant lobes. Before the epic event, the stellar pair was 140 times heftier than our Sun.
Because Eta Carinae is relatively nearby, astronomers have used a variety of telescopes, including the Hubble Space Telescope, to document its escapades. The team's study involved a mix of visible-light and spectroscopic observations from ground-based telescopes.
The observations mark the first time astronomers have used spectroscopy to analyze a light echo from a star undergoing powerful recurring eruptions, though they have measured this unique phenomenon around exploding stars called supernovae. Spectroscopy captures a star's "fingerprints," providing details about its behavior, including the temperature and speed of the ejected material.
The delayed broadcast is giving astronomers a unique look at the outburst and turning up some surprises. The turbulent star system does not behave like other stars of its class. Eta Carinae is a member of a stellar class called Luminous Blue Variables, large, extremely bright stars that are prone to periodic outbursts. The temperature of the outflow from Eta Carinae's central region, for example, is about 8,500 degrees Fahrenheit (5,000 Kelvin), which is much cooler than that of other erupting stars. "This star really seems to be an oddball," Rest said. "Now we have to go back to the models and see what has to change to actually produce what we are measuring."
Rest's team first spotted the light echo while comparing visible-light observations he took of the stellar duo in 2010 and 2011 with the U.S. National Optical Astronomy Observatory's Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. He obtained another set of CTIO observations taken in 2003 by astronomer Nathan Smith of the University of Arizona in Tucson, which helped him piece together the whole 20-year outburst.
The images revealed light that seemed to dart through and illuminate a canyon of dust surrounding the doomed star system. "I was jumping up and down when I saw the light echo," said Rest, who has studied light echoes from powerful supernova blasts. "I didn't expect to see Eta Carinae's light echo because the eruption was so much fainter than a supernova explosion. We knew it probably wasn't material moving through space. To see something this close move across space would take decades of observations. We, however, saw the movement over a year's time. That's why we thought it was probably a light echo."
Although the light in the images appears to move over time, it's really an optical illusion. Each flash of light is reaching Earth at a different time, like a person's voice echoing off the walls of a canyon.
The team followed up its study with spectroscopic observations, using the Carnegie Institution of Washington's Magellan and du Pont telescopes at Las Campanas Observatory in Chile. That study helped the astronomers decode the light, revealing the outflow's speed and temperature. The observations showed that ejected material was moving at roughly 445,000 miles an hour (more than 700,000 kilometers an hour), which matches predictions.
Rest's group monitored changes in the intensity of the light echo using the Las Cumbres Observatory Global Telescope Network's Faulkes Telescope South in Siding Spring, Australia. The team then compared those measurements with a plot astronomers in the 1800s made of the light brightening and dimming over the course of the 20-year eruption. The new measurements matched the signature of the 1843 peak in brightness.
The team will continue to follow Eta Carinae because light from the outburst is still streaming to Earth. "We should see brightening again in six months from another increase in light that was seen in 1844," Rest said. "We hope to capture light from the outburst coming from different directions so that we can get a complete picture of the eruption."
Rest's team consists of J.L. Prieto, Carnegie Observatories, Pasadena, Calif.; N.R. Walborn and H.E. Bond, Space Telescope Science Institute, Baltimore, Md.; N. Smith, Steward Observatory, University of Arizona, Tucson; F.B. Bianco and D.A. Howell, Las Cumbres Observatory Global Telescope Network, Goleta, Calif., and University of California, Santa Barbara; R. Chornock, R.J. Foley, and W. Fong, Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.; D.L. Welch and B. Sinnott, McMaster University, Hamilton, Ontario; M.E. Huber, Johns Hopkins University, Baltimore, Md.; R.C. Smith, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, La Serena, Chile; I. Toledo, Atacama Large Millimeter Array (ALMA), Chile; D. Minniti, Pontifica Universidad Catolica, Santiago, Chile; and K. Mandel, Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., and Imperial College London, U.K.

Plasmas Torn Apart: Discovery Hints at Origin of Phenomena Like Solar Flares

January saw the biggest solar storm since 2005, generating some of the most dazzling northern lights in recent memory.


he source of that storm -- and others like it -- was the sun's magnetic field, described by invisible field lines that protrude from and loop back into the burning ball of gas. Sometimes these field lines break -- snapping like a rubber band pulled too tight -- and join with other nearby lines, releasing energy that can then launch bursts of plasma known as solar flares. Huge chunks of plasma from the sun's surface can zip toward Earth and damage orbiting satellites or bump them off their paths.
These chunks of plasma, called coronal mass ejections, can also snap Earth's magnetic field lines, causing charged particles to speed toward Earth's magnetic poles; this, in turn, sets off the shimmering light shows we know as the northern and southern lights.
Even though the process of field lines breaking and merging with other lines -- called magnetic reconnection -- has such significant effects, a detailed picture of what precisely is going on has long eluded scientists, says Paul Bellan, professor of applied physics in the Division of Engineering and Applied Science at the California Institute of Technology (Caltech).
Now, using high-speed cameras to look at jets of plasma in the lab, Bellan and graduate student Auna Moser have discovered a surprising phenomenon that provides clues to just how magnetic reconnection occurs. They describe their results in a paper published in the February 16 issue of the journal Nature.
"Trying to understand nature by using engineering techniques is indeed a hallmark of the Division of Engineering and Applied Science at Caltech," says Ares Rosakis, the Theodore von Kármán Professor of Aeronautics and professor of mechanical engineering and the chair of engineering and applied science.
In the experiments, Moser fired jets of hydrogen, nitrogen, and argon plasmas at speeds of about 10 to 50 kilometers per second across a distance of more than 20 centimeters in a vacuum. Plasma is a gas so hot that atoms are stripped of their electrons. As a throughway for speeding electrons, the jets act like electrical wires. The experiment requires 200 million watts of power to produce jets that are a scorching 20,000 degrees Kelvin and carry a current of 100,000 amps. To study the jets, Moser used cameras that can take a snapshot in less than a microsecond, or one millionth of a second.
As in all electrical currents, the flowing electrons in the plasma jet generate a magnetic field, which then exerts a force on the plasma. These electromagnetic interactions between the magnetic field and the plasma can cause the jet to writhe and form a rapidly expanding corkscrew. This behavior, called a kink instability, has been studied for nearly 60 years, Bellan says.
But when Moser looked closely at this behavior in her experimental plasma jets, she saw something entirely unexpected.
She found that -- more often than not -- the corkscrew shape that developed in her jets grew exponentially and extremely fast. The jets in the experiment formed 20-centimeter-long coils in just 20 to 25 microseconds. She also noticed tiny ripples that began appearing on the inner edge of the coil just before the jet broke -- the moment when there was a magnetic reconnection.
In the beginning, Moser and Bellan say, they did not know what they were seeing -- they just knew it was strange. "I thought it was a measurement error," Bellan says. "But it was way too reproducible. We were seeing it day in and day out. At first, I thought we would never figure it out."
But after months of additional experiments, they determined that the kink instability actually spawns a completely different kind of phenomenon, called a Rayleigh-Taylor instability. A Rayleigh-Taylor instability happens when a heavy fluid that sits on top of a light fluid tries to trade places with the light fluid. Ripples form and grow at the interface between the two, allowing the fluids to swap places.
What Moser and Bellan realized is that the kink instability creates conditions that give rise to a Rayleigh-Taylor instability. As the coiled plasma expands -- due to the kink instability -- it accelerates outward. Just like a passenger being pushed back into the seat of an accelerating car, the accelerated plasma is pushed down on the vacuum behind it. The plasma tries to swap places with the trailing vacuum by forming ripples that then expand -- just like when gravity forces a heavy fluid to try to change places with a light fluid underneath. The Rayleigh-Taylor instability -- as revealed by the ripples on the trailing side of the accelerating plasma -- grows in about a microsecond.
"People have not observed anything like this before," Bellan says.
Although the Rayleigh-Taylor instability has been studied for more than 100 years, no one had considered the possibility that it could be caused by a kink instability, Bellan says. The two types of instabilities are so different that to see them so closely coupled was a shock. "Nobody ever thought there was a connection," he says.
What is notable is that the two instabilities occur at very different scales, the researchers say. While the coil created by the kink instability spans about 20 centimeters, the Rayleigh-Taylor instability is much smaller, making ripples just two centimeters long. Still, those smaller ripples rapidly erode the jet, forcing the electrons to flow faster and faster through a narrowing channel. "You're basically choking it off," Bellan explains. Soon, the jet breaks, causing a magnetic reconnection.
Magnetic reconnection on the sun often involves phenomena that span scales from a million meters to just a few meters. At the larger scales, the physics is relatively simple and straightforward. But at the smaller scales, the physics becomes more subtle and complex -- and it is in this regime that magnetic reconnection takes place. Magnetic reconnection is also a key issue in developing thermonuclear fusion as a future energy source using plasmas in the laboratory. One of the key advances in this study, the researchers say, is being able to relate phenomena at large scales, such as the kink instability, to those at small scales, such as the Rayleigh-Taylor instability.
The researchers note that, although kink and Rayleigh-Taylor instabilities may not drive magnetic reconnection in all cases, this mechanism is a plausible explanation for at least some scenarios in nature and the lab.
The title of Moser and Bellan's Nature paper is "Magnetic reconnection from a multiscale instability cascade." This research was funded by the U.S. Department of Energy, the National Science Foundation, and the Air Force Office of Scientific Research.

Newborn Stars Emerge from Dark Clouds in Taurus

A new image from the APEX (Atacama Pathfinder Experiment) telescope in Chile shows a sinuous filament of cosmic dust more than ten light-years long. In it, newborn stars are hidden, and dense clouds of gas are on the verge of collapsing to form yet more stars. It is one of the regions of star formation closest to us. The cosmic dust grains are so cold that observations at wavelengths of around one millimetre, such as these made with the LABOCA camera on APEX, are needed to detect their faint glow.


The Taurus Molecular Cloud, in the constellation of Taurus (The Bull), lies about 450 light-years from Earth. This image shows two parts of a long, filamentary structure in this cloud, which are known as Barnard 211 and Barnard 213. Their names come from Edward Emerson Barnard's photographic atlas of the "dark markings of the sky," compiled in the early 20th century. In visible light, these regions appear as dark lanes, lacking in stars. Barnard correctly argued that this appearance was due to "obscuring matter in space."
We know today that these dark markings are actually clouds of interstellar gas and dust grains. The dust grains -- tiny particles similar to very fine soot and sand -- absorb visible light, blocking our view of the rich star field behind the clouds. The Taurus Molecular Cloud is particularly dark at visible wavelengths, as it lacks the massive stars that illuminate the nebulae in other star-formation regions such as Orion. The dust grains themselves also emit a faint heat glow but, as they are extremely cold at around -260 degrees Celsius, their light can only be seen at wavelengths much longer than visible light, around one millimetre.
These clouds of gas and dust are not merely an obstacle for astronomers wishing to observe the stars behind them. In fact, they are themselves the birthplaces of new stars. When the clouds collapse under their own gravity, they fragment into clumps. Within these clumps, dense cores may form, in which the hydrogen gas becomes dense and hot enough to start fusion reactions: a new star is born. The birth of the star is therefore surrounded by a cocoon of dense dust, blocking observations at visible wavelengths. This is why observations at longer wavelengths, such as the millimetre range, are essential for understanding the early stages of star formation.
The upper-right part of the filament shown here is Barnard 211, while the lower-left part is Barnard 213. The millimetre-range observations from the LABOCA camera on APEX, which reveal the heat glow of the cosmic dust grains, are shown here in orange tones, and are superimposed on a visible light image of the region, which shows the rich background of stars. The bright star above the filament is φ Tauri, while the one partially visible at the left-hand edge of the image is HD 27482. Both stars are closer to us than the filament, and are not associated with it.
Observations show that Barnard 213 has already fragmented and formed dense cores -- as illustrated by the bright knots of glowing dust -- and star formation has already happened. However, Barnard 211 is in an earlier stage of its evolution; the collapse and fragmentation is still taking place, and will lead to star formation in the future. This region is therefore an excellent place for astronomers to study how Barnard's "dark markings of the sky" play a crucial part in the lifecycle of stars.
The observations were made by Alvaro Hacar (Observatorio Astronómico Nacional-IGN, Madrid, Spain) and collaborators. The LABOCA camera operates on the 12-metre APEX telescope, on the plateau of Chajnantor in the Chilean Andes, at an altitude of 5000 metres. APEX is a pathfinder for the next generation submillimetre telescope, the Atacama Large Millimeter/submillimeter Array (ALMA), which is being built and operated on the same plateau.

Globular Clusters: Survivors of a 13-Billion-Year-Old Massacre

Our Milky Way galaxy is surrounded by some 200 compact groups of stars, containing up to a million stars each. At 13 billion years of age, these globular clusters are almost as old as the universe itself and were born when the first generations of stars and galaxies formed. Now a team of astronomers from Germany and the Netherlands have conducted a novel type of computer simulation that looked at how they were born -- and they find that these giant clusters of stars are the only survivors of a 13-billion-year-old massacre that destroyed many of their smaller siblings.
The new work, led by Dr Diederik Kruijssen of the Max Planck Institute for Astrophysics in Garching, Germany, appears in a paper in the journal Monthly Notices of the Royal Astronomical Society.
Globular star clusters have a remarkable characteristic: the typical number of stars they contain appears to be about the same throughout the Universe. This is in contrast to much younger stellar clusters, which can contain almost any number of stars, from fewer than 100 to many thousands. The team of scientists proposes that this difference can be explained by the conditions under which globular clusters formed early on in the evolution of their host galaxies.
The researchers ran simulations of isolated and colliding galaxies, in which they included a model for the formation and destruction of stellar clusters. When galaxies collide, they often generate spectacular bursts of star formation (“starbursts”) and a wealth of bright, young stellar clusters of many different sizes. As a result it was always thought that the total number of star clusters increases during starbursts. But the Dutch-German team found the opposite result in their simulations.
While the very brightest and largest clusters were indeed capable of surviving the galaxy collision due to their own gravitational attraction, the numerous smaller clusters were effectively destroyed by the rapidly changing gravitational forces that typically occur during starbursts due to the movement of gas, dust and stars. The wave of starbursts came to an end after about 2 billion years and the researchers were surprised to see that only clusters with high numbers of stars had survived. These clusters had all the characteristics that should be expected for a young population of globular clusters as they would have looked about 11 billion years ago.
Dr Kruijssen comments: “It is ironic to see that starbursts may produce many young stellar clusters, but at the same time also destroy the majority of them. This occurs not only in galaxy collisions, but should be expected in any starburst environment. In the early Universe, starbursts were commonplace – it therefore makes perfect sense that all globular clusters have approximately the same large number of stars. Their smaller brothers and sisters that didn’t contain as many stars were doomed to be destroyed.”
According to the simulations, most of the star clusters were destroyed shortly after their formation, when the galactic environment was still very hostile to the young clusters. After this episode ended, the surviving globular clusters have lived quietly until the present day.
The researchers have further suggestions to test their ideas. Dr Kruijssen continues: “In the nearby Universe, there are several examples of galaxies that have recently undergone large bursts of star formation. It should therefore be possible to see the rapid destruction of small stellar clusters in action. If this is indeed found by new observations, it will confirm our theory for the origin of globular clusters.”
The simulations suggest that most of a globular cluster’s traits were established when it formed. The fact that globular clusters are comparable everywhere then indicates that the environments in which they formed were very similar, regardless of the galaxy they currently reside in. In that case, Dr Kruijssen believes, they can be used as fossils to shed more light on the conditions in which the first stars and galaxies were born.

The above story is reprinted from materials provided by Royal Astronomical Society (RAS), via AlphaGalileo.